Field of the Invention
The present invention relates to an alloy that is resistant to oxidation and/or corrosion, in particular at high temperature. More particularly, the invention relates to an alloy resistant to oxidation in a corrosive environment, such as molten glass or another similar material, which can be used to make articles which are brought into contact with molten glass or similar material when it is being prepared or hot converted.
2. Discussion of the Background
In the glass industry, the elements or tools in contact with the molten glass are made of refractory materials, in particular ceramics. For certain operations, it is preferable to use metal tools, generally made of alloy, in particular when the tool needs to have electrical conduction properties or when the tool needs to have a certain ductility and mechanical strength at the temperature at which the molten glass is processed.
This is the case, for example, for the manufacture of glass wool by the technique referred to as internal centrifugation, more particularly as regards the final phase of the process in which, on leaving the melting furnace, the glass is cast continuously in a set of axisymmetric elements rotating at a very high rotational speed about their vertical axis. Stopped in its initial fall by the bottom of an internal part, or "basket", the glass spreads under the effect of the centrifugal force against the cylindrical wall of the same part, this wall being pierced by holes. These holes allow the glass to pass through and, still under the effect of the centrifugal force, it becomes pressed against the wall, or "band", of an external part, or "spinner", which is also pierced with holes, these holes being smaller than the previous ones. Still under the effect of the centrifugal force, the glass passes through the band of the spinner from all sides in the form of molten glass filaments. An annular burner located above the outside of the spinner, producing a downward gas stream running along the outer wall of the band, deflects these filaments downward while stretching them. They then "solidify" in the form of glass wool.
The parts referred to as "basket" and "spinner" are fiber processing tools subjected to very high degrees of stress thermally (thermal shocks when starting and stopping), mechanically (centrifugal force, erosion due to the transit of the glass) and chemically (oxidation and corrosion by the molten glass, and by the hot gases leaving the burner, in the case of the spinner). Typically, the operating temperature is on the order of at least 1000.degree. C., so that the glass has a suitable viscosity.
The life of these components generally depends on the corrosion resistance of the material of which they are made. In this regard, use is generally made of a nickel-based alloy containing about 30% chromium and reinforced by carbide precipitation, as described especially in FR-A-2 536 385.
The oxidation and corrosion resistance of this alloy at the temperature at which it is used is ensured by its high proportion of chromium, which forms a protective layer of chromia (chromium oxide, Cr.sub.2 O.sub.3) at the surface of the part in contact with the oxidizing environment. Continuous diffusion of chromium toward the corrosion front makes it possible to keep a chromium reserve behind the layer of oxides, e.g., Cr.sub.2 O.sub.3.
The working temperatures at which this alloy can be used successfully are, however, limited to a maximum value of the order of 1050 to 1100.degree. C. Above this temperature, the material is rapidly degraded by corrosion as well as by creeping. This material is therefore incapable of meeting the demand of techniques by which wool is produced from glasses that are more viscous than the customary glasses of the borosilicate type and whose use requires temperatures in excess of 1100.degree. C.
In order to meet this need, one object of the present invention is to provide an alloy having improved resistance to corrosion, and more particularly to oxidation at high temperature, and in particular up to temperatures of the order of 1300.degree. C.
Certain alloys based on metals more refractory than nickel are known to have good resistance to corrosion by glass at elevated temperature. Mention may in particular be made of a cobalt-based alloy according to FR-A-2 273 075.
Other special alloys, such as alloys reinforced by oxide dispersion (ODS), superalloys in general based on nickel and/or iron containing, in dispersion within their matrix, fine particles of oxides, and generally yttrium oxide, have also been used to make fiber processing spinners having good mechanical properties at elevated temperature. An alloy of this type is described, in particular, in U.S. Pat. No. 5,328,499.
It is, however, difficult with known alloys to achieve temperatures in excess of 1200-1250.degree. C. while having an oxidation strength compatible with industrial production requirements.
As an alternative, it is known to use highly refractory metals, such as molybdenum or tungsten, which are endowed with high resistance to corrosion by glass at high temperature when they are fully immersed in the glass. The problem that these refractory metals have in common is still, however, their sensitivity to atmospheres containing oxygen. This is because their reactivity is actually very high and leads to the formation of oxides which are either poor protectors, as in the case of tantalum or niobium, or are highly volatile, as in the case of molybdenum and tungsten, and responsible for rapid degradation by corrosion at high temperature.
Accordingly, another object of the present invention is to improve the oxidation and/or corrosion strength of a metal or metal alloy at high temperature.